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Re: Slab on grade

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To : Phil Hodge

Some follow up comments to your questions:

1. fs is the working stress of the WWF which should be 2/3 to 3/4 of the
60,000 psi. to remain elastic. 40,000 psi is commonly used. This
equation has been around for years and was first published by the Wire
Reinforcement Institute in 1973, "Design Procedures for Industrial
Slabs". The equation assumes that the slab can shrink from each end (not
the same case if one end is thickened).

2. The subgrade drag coefficient is commonly assumed to be 1.5. PTI's
publication "Design of Post-Tensioned Slabs on Ground" has a table with
other values. I don't have a copy of this, but if someone does, I would
appreciate a fax is (920) 437-1131. Incidently, by putting
down one or two layers of visqueen, you can reduce the subgrade drag to
very little.......this is what is typically done for post-tensioned
slabs-on-grade. I don't, however, recommend that for your project.

3. I would use 5/8" diameter dowels @ 24" o.c. instead of 1/2" @ 12"o.c.
It will reduce your labor costs of setting all those dowels. The dowels
will cause short shrinkage cracks to form at right angles to the cold
joint when the other slab tries to shrink and the first slab restrains
it. Add a couple #4's parallel to the joint in the 2nd slab. This will
intercept these cracks.

Another alternative is to use those square dowels that come with the
foam on each side. I don't remember the name or the manufacturer, but it
is designed to transfer the vertical shear forces without restraining
the shrinkage of the slab.

4. I don't recommend fibers since you have plenty of steel and don't
need to spend the additional money.

5. I don't know enough of how the superstructure is supported to comment
on the thickened slab edge. Is there a separate foundation wall and
piers and footings or are there footings poured integrally with the

Generally, I would advise against a thickened edge, if the slab butts up
to a foundation wall. Restraint of the slab shrinkage is a concern. The
thickened edge is only required if it is possible to have heavy
concentrated loads near the edge. In most buildings, this is unlikely,
except at doors. A beam on elastic foundation analysis can confirm this.

If the column footings are poured integrally with the slab, then I would
have a thickened edge to help distribute loads and to prevent
undermining of the edge.

Good Luck!

Jim Kestner
Green Bay, Wi